This invention relates to an ophthalmic instrument for use in testing the vision of a subject.
A known type of instrument for this purpose briefly displays to the subject predetermined patterns of spots of light, the subject indicates what he has seen of these patterns, and from his observations of a number of successive patterns it is possible to characterise particular deficiencies in his field of vision. Examples of such an instrument, known commercially as a Friedmann Analyser, are described in British patent specifications Nos. 925 066 and 2 026 197A.
Because each display pattern is shown only briefly to the subject, e.g. for 1/500th of a second, in order to ensure the subject does not scan the field of vision each time but focusses on a fixed point, only a very limited number of points of light can be included in each pattern, e.g. not more than 4 points each time. This means that a large number of test patterns must be displayed to cover the field of view being examined. In order to assist the task of the operator in recording the results of these many tests, the specification of British patent application No. 2 026 197A describes how the patterns to be displayed can be shown on a screen that is visible only to the operator and not the subject, and the results of the successive tests can be recorded on a chart mounted over that screen.
While the provision of monitoring display for the operator considerably simplifies his task and helps to avoid recording errors, it is found in practice that the means for producing the screen display is not always reliable and that the quality of the screen display is not easily controlled. The present invention is particularly concerned with improving the instrument in these respects.
SUMMARY OF THE INVENTION
According to the invention, in an ophthalmic instrument for displaying to a subject different patterns of light spots in a series of visual tests, the instrument comprising an apertured shutter arrangement for determining the pattern of light spots to be displayed, a member of said shutter arrangement being displaceable to select different light patterns and a screen or like display area being provided on the instrument for displaying to the operator but not the subject each light pattern selected, the means for producing the pattern on said screen comprises a series of light-emitting elements in fixed positions over the area of the screen, said elements being actuated by the use of radiation transmitting means which are arranged to transmit radiation onto reflecting means from which reflected radiation is received by sensing means in dependence upon the movement of the displaceable shutter member, said reflecting means transmitting to said sensing means a different radiation reflection characteristic for each light-spot pattern displayed at the first location and said sensing means responding to the reflected radiation characteristic to actuate the light-emitting elements corresponding to the selected pattern.
In one form of the invention, the reflecting means is movable with the displaceable shutter member for transmission to the sensing means the different reflected radiation characteristics associated with the different light-spot patterns. Additionally or alternatively, there may be masking means for radiation transmission between the transmitting and sensing means movable with the displaceable member for selective exposure of the reflecting means to the radiation.
By producing the operator's screen patterns in these ways, a major source of difficulty is avoided in that the active elements of the means for producing the pattern can all be mounted in fixed positions and the only movable elements are the passive elements that determine the reflection characteristic of the radiation. It is therefore possible to avoid the use of relatively movable electrical contacts which are prone to malfunction over an extended period of use.
According to a preferred feature of the invention, a considerable simplification of the radiation and sensing means is possible by arranging the reflecting means in the form of a coded pattern whereby a series of different reflection characteristics can be generated using a lesser number of radiation sensing means than the number of patterns to be displayed in a test series. As has already been mentioned, a large number of individual patterns must be shown to the subject because of the need to limit the number of light spots in each pattern. A standard series of tests for vision involves a total of 31 light spot patterns and, employing the fibre optics arrangement of patent application No. 2 026 197A for producing the pattern on the operator's screen, a corresponding number of fibre bundles must be provided.
If, however, said reflecting means is arranged to produce a binary-coded reflection characteristic, all the alternatives can be identified using a five-character binary code, so that only five radiation sensing elements are necessary. This can lead to a considerable simplification and, in particular, because the total angular movement of the displaceable shutter member is limited, it makes it easier to dispose the radiation sensing means in a way that reduces any risk of spurious signals through misalignment.
As a convenient and reliable arrangement, infra-red radiation transmitting and sensing elements can be used in the form of units in each of which a sensing element is adapted to receive reflected radiation from an integral emitting element. Radiation frequencies other than in the infra-red range can of course also be employed.
The reflecting means may be economically produced as a patterned coating on one member of the apertured shutter arrangement. Such a coating may be formed by a conventional printed circuit technique and given a final highly reflective surface finish; other possibilities include hot stamping, or direct deposition of a highly reflective material directly onto the shutter surface.
The invention will be described by way of example with reference to the accompanying drawings.